Directional drilling is the process of steering a drill string, and hence the borehole. It can be achieved with a variety of drill string steering mechanisms, e.g., whipstocks, mud motors with bent-housings, jetting bits, adjustable gauge stabilizers, and rotary steering systems (RSS). Each of these mechanisms employs side force, bit tilt angle, or some combination thereof, to steer the drill string's forward and rotary motion. They may be used to avoid obstacles and reach desired targets, both of which may take various forms. For example, a target may be specified in terms of an entry point to a formation, together with a desired entry vector. Both the entry point and vector may be specified as ranges or accompanied by acceptable tolerances. Some boreholes may even be associated with a series of such entry points and vectors.
Drillers generally employ careful trajectory planning not only to ensure that targets are reached and obstacles avoided, but also to limit curvature and tortuosity of the borehole. Such limits are needed to prevent the drill string and other tubulars from getting stuck, to avoid excessive friction, and to minimize casing wear.
Trajectory planning is generally subject to information uncertainty from a number of sources. For example, the drill string continuously encounters formations whose precise properties are often not known in advance, but which affect the operation of the bit, or more precisely, affect the operating parameter ranges that induce bit whirl, stick-slip, vibration, and other undesirable behaviors, as well as affecting the relationship between those parameters and the ROP. The drilling dynamics model used to predict such behaviors might be mismatched with the physical drill string assembly. The formation heterogeneity may also be uncertain, as well as the precise positions of the formation boundaries and any detected formation anomalies. The operating parameters themselves may not be precisely known (e.g., rotations per minute (RPM), torque, hook load, weight on bit (WOB), downhole pressure, drilling fluid flow rate), whether due to inaccuracies in the control mechanisms or sensor noise. The steering mechanism may suffer from bit walk or other steering inaccuracies.
It is in this context of uncertainty that drillers must operate as they seek to reach their targets quickly and efficiently. Achieving this goal requires consideration of a surprising range of factors including rate of penetration (ROP), likelihood of equipment damage and commensurate non-productive time (NPT) spent on repairs, and other sources of NPT including tripping the bottomhole assembly (BHA), replacing worn bits, performing survey measurements, and recalibrating drilling subsystems. Drillers lack sufficient guidance for optimizing their operating parameters in the face of such complexity and uncertainty.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.